Despite progress in unraveling the mechanism of the HIV replication cycle in human cells, we have limited understanding of how virus is transmitted in vivo, how it causes immunodeficiency, and why immune responses are generally ineffective in neutralizing it. These issues are difficult to study in humans and even in non-human primates. Our broad aim is to generate a murine model system that will faithfully reproduce the interactions of HIV- 1 with its human host. This would allow for relatively inexpensive examination of various aspects of viral transmission, replication, and pathogenesis with numerous variables, and it would permit studies of candidate host genetic factors in HIV disease. Development of such a model will require a better understanding of how virus is transmitted and of species-specific restrictions that limit HIV replication in murine cells. In vitro studies suggest that dendritic cells (DC) have important roles in HIV transmission and replication. DC endocytose HIV particles, harbor HIV for long periods of time, and enhance infection of T cells in trans. DC-SIGN is a key DC receptor for HIV endocytosis, a process required for efficient HIV replication in DC:T cell co-cultures. The importance of HIV interaction with DC has not yet been tested in vivo. Specific Aim 1 will be to use genetically modified mice to determine if HIV can be transmitted to cells in draining lymphatic tissues after exposure to DC within mucosa. We will examine mice transgenic for DC-SIGN in combination with SCID-Hu mice and mice expressing human CD4, CCR5, and cyclin T1. Although T cells and macrophages from the multiply transgenic animals can be infected with HIV, they fail to release infectious particles. Specific Aim 2 will be to employ genetic approaches to identify human genes that enhance HIV assembly and/or release from murine cells and that will permit better replication of HIV in the murine model. Specific Aim 3 will be to determine if APOBEC3G, an antiviral protein targeted for degradation by the HIV Vif protein, restricts replication in primary mouse cells. If so, we will develop means to overcome its activity. Vif induces degradation of human, but not mouse, APOBEC3G. RNA interference and gene targeting will be used to reduce expression of the mouse enzyme, and we will engineer HIV with Vif that can bind to and induce degradation of mouse apobec3g. [unreadable] [unreadable]